Neurodegenerative diseases are a significant cause of morbidity and mortality in elderly Veterans. No cures exist for neurodegenerative diseases, because the cellular, molecular, and genetic mechanisms that cause neurons to die with age are poorly understood. Our long-term objective is to decipher the mechanisms required for neuronal function and survival and to identify how dysfunction of these mechanisms contribute to progressive age-associated neurodegenerative diseases. Comprehensive genome wide studies of patients with late-onset Alzheimer's disease and Progressive Supranuclear Palsy recently identified the eukaryotic translation initiation factor 2? kinase 3/PKR-like endoplasmic reticulum kinase (EIF2AK3/PERK) gene as a genetic risk factor for neurodegeneration. EIF2AK3/PERK encodes an endoplasmic reticulum transmembrane protein kinase that is essential for cells to survive pathologic and environmental conditions that cause misfolded proteins and endoplasmic reticulum stress. In response to misfolded proteins and ER stress, EIF2AK3/PERK dimerizes to activate its kinase domain. Multiple EIF2AK3/PERK haplotypes with various amino acid substitutions are found in the human population. The function of EIF2AK3/PERK in neurons and the mechanism by which some EIF2AK3/PERK haplotypes cause neurodegeneration are unknown. We recently discovered that EIF2AK3/PERK haplotypes associated with neurodegeneration have significantly reduced kinase activity compared to protective haplotypes. We also found that neurons generated from skin fibroblasts of patients with high-risk EIF2AK3/PERK haplotypes showed impaired EIF2AK3/PERK signaling in response to ER stress. Last, we found that EIF2AK3/PERK signaling is robustly activated in a mouse model of tauopathy neurodegeneration. Based on these preliminary findings, our central hypothesis is that EIF2AK3/PERK is activated during neurodegeneration to preserve neuronal cell function and viability, and loss of EIF2AK3/PERK function leads to increased protein misfolding and increased ER stress that ultimately cause neuronal cell death and neurodegeneration. We propose cellular, molecular, and genetic experiments to test this hypothesis and determine the function of EIF2AK3/PERK in neurodegeneration. First, we will characterize the biochemical and enzymatic activities of high-risk and low-risk human EIF2AK3/PERK associated with neurodegeneration. We will perform biochemical studies of recombinant EIF2AK3/PERK proteins to analyze the functional consequences of amino acid alterations associated with human haplotype variants. Second, we will test if modulation of EIF2AK3/PERK signaling prevents disease in the PS19 transgenic mouse model of neurodegeneration. We will cross PS19 mice with genetically modified mice that increase EIF2AK3/PERK signaling. We will treat PS19 mice with orally available drugs that increase or inhibit EIF2AK3/PERK signaling. We will perform molecular, histologic, and behavioral studies to examine how EIF2AK3/PERK modulation affects neuropathology and neurodegeneration in this animal model. Last, we will evaluate EIF2AK3/PERK signal transduction in stem cell-derived neurons generated from patients with high-risk and low-risk EIF2AK3/PERK haplotypes. We will determine how genetic variants of EIF2AK3/PERK affect neuronal tau protein folding and neuronal susceptibility to ER stress-induced cell death. In parallel, we will test novel pharmacologic modulators of EIF2AK3/PERK signaling to see if we can prevent neuronal cell death when PERK is artificially activated. In summary, EIF2AK3/PERK variants are important genetic risk factors for developing neurodegeneration but their function and mechanisms are unknown and treatments are lacking. These studies are significant because they will reveal the role of EIF2AK3/PERK in neurons and why EIF2AK3/PERK variants in people can increase risk for developing neurodegeneration. Our studies will positively benefit the mission of the VA by identifying potential treatments to prevent neurodegeneration in Veterans and patients carrying the high-risk alleles.